The present invention relates to non-destructive testing of conductive materials, namely eddy-current testing. More particularly, the invention relates to eddy current testing (ECT) probes, for example of the type suitable for inspecting heat exchanger tubes.
ECT probes are well known in the art, and have various configurations depending on the nature of the material being tested. Some ECT probes are adapted for testing planar surfaces, others for testing tubes. In the case of tubes, both the interior and the exterior may be tested, and the probes are suitably adapted accordingly. ECT probes, in the case of heat exchanger tube inspection probes for inspecting the insides of heat exchanger tubes, have a probe head with coils for inducing and detecting eddy currents in the conductive material being tested. Coils may be configured in arrangements known as xe2x80x9cbraceletsxe2x80x9d of coils, namely series of coils on the circumferential surface of the probe head. A bracelet may be provided as circumferential bands consisting of many coils.
The instrumentation for ECT consists of the probe including the probe head having the coils, signal generator and receiver equipment, the cabling connecting the probe to the signal generator and receiver equipment, and the signal analyzer equipment for analyzing the data and providing an indication of faults in the material being tested. The probe and cabling is typically subjected to extreme industrial environments which place considerable physical stress on the equipment. In the case of tube inspection, the cabling includes often a flexible shaft or tube for inserting the probe into the heat exchanger tubes, and the electrical signal conductors must therefore be durable, reliable and as thin as possible to fit inside the flexible shaft, and also to lessen the burden of cable manipulation from the signal processing equipment and the probe.
A common ECT technique to inspect tubes is to use a bobbin coil. This type of probe examines an entire cross-section of the tube at once. The difficulty is to detect small volume flaws, long axial flaws, and circumferential flaws, anywhere on either the inside or outside of the tube, and especially where the tubes are expanded.
An improved technique to the detect flaws described above is the use of probes that use one or a few small rotating coil, scanning the surface of the tube. The small coil or coils only look at a small portion of the tube surface at any given time, increasing sensitivity. Because of the mechanical rotation, such probes are typically slow.
The use of an array of coils, such as a bracelet, ensures that only small surface areas are inspected with each coil, resulting in sensitivity similar to that of the rotating probes, but without requiring mechanical rotation. Array probes, when coupled with adequate data acquisition systems provide excellent sensitivity at much greater speeds than rotating probes.
Furthermore, providing a very large number of coils in an effort to satisfy the requirement for quality of detection also results in a large number of conductors being required for carrying the coil signals. While it is possible to use smaller wires, smaller wires are more fragile, have poorer transmission characteristics, and as they are brought closer together, they have greater crosstalk.
It is an object of the present invention to provide an ECT probe which overcomes many of the drawbacks associated with known ECT probes.
More particularly, it is a first object of the present invention to provide an ECT probe which uses an arrangement of coils in different measurement configurations so as to involve coil reuse. By xe2x80x9ccoil reusexe2x80x9d, it is meant that at least one coil from one measurement configuration is also used in another measurement configuration with at least some different coils. According to this aspect of the invention a coil may be used during one measurement as a transmit coil, and during a subsequent measurement as a receive coil. Likewise, one of two coils in a differential mode configuration during one measurement may be used in a subsequent measurement in a differential mode configuration with a different other coil, or as an absolute coil. Preferably, an array of closely arranged coils is provided and the coils are used in different measurement configurations, so as to detect faults in the surface of the material being tested in a greater number of positions (i.e. a higher density of inspection coverage), and possibly directions.
It is a second object of the present invention to provide an ECT probe which reduces the number of conductors connecting the probe coils to the instrumentation by selectively connecting coils to conductors at the probe end (or routing coil signals onto conductors). Preferably, such selective connection is done using a multiplexer switch controlled by a selection signal. Using solid state switches high selection rates are possible, and high speed scanning is possible.
Preferably, coils not being used in a measurement are disconnected or isolated using devices, such as separate solid state switches.
According to a broad aspect of the invention, there is provided a method of detecting faults in a conductive material to be tested using eddy currents, the method comprising:
a) providing a probe head having an arrangement of coils arranged on a surface of the probe to induce eddy currents in the conductive material to be tested, wherein the arrangement allows the probe to cover an area transverse to a direction of displacement of the probe head during inspection;
b) selectively exciting one of the coils with an AC source to induce an eddy current in the material;
c) measuring eddy current flow in the conductive material to be tested using at least one of said coils;
d) repeating steps b) and c) to cover the area transverse to the direction of displacement in different measurement configurations in which at least one of:
i. one of the coils used as a receiver coil is reused as a driven coil;
ii. the excited one of the coils is reused as a receiver coil;
iii. one of two or more coils used as receiver coils in a differential mode is used as a receiver coil with a different other receiver coil;
iv. a same transmit conductor is used for connecting the AC source to the probe head, step b) comprising switching the excited coil to the transmit conductor; and
v. a same receiver conductor is used for connecting a receiver to the probe head, step c) comprising switching one of the at least one of the coils to the receiver conductor.